1. Field of the Invention
The present invention generally relates to a color laser printer using an electro-photographic method, and more specifically, to an apparatus to transfer a visible image onto printing paper by a toner formed on a photosensitive medium.
2. Description of the Related Art
A conventional color laser printer using an electro-photographic method develops an electrostatic latent image formed on a photosensitive medium according to an image signal to a visible color image using toners of yellow, magenta, cyan and black. Then, the color laser printer using the electro-photographic method transfers and prints the color image onto the printing paper using a transferring apparatus.
The above-mentioned transferring apparatus includes an intermediate transfer belt 10, a transfer roller 20 and a transfer roller separating/pressurizing unit 30, as illustrated in FIGS. 1, 2A and 2B.
The intermediate transfer belt 10 is provided to travel along a loop, the intermediate transfer belt 10 being wound about a plurality of guide rollers 11-15 disposed on a printer body frame (not shown). The intermediate transfer belt 10 is connected to a photosensitive drum 1 as a photosensitive medium by the guide roller 12, and thereby a toner image of the photosensitive drum 1 is transferred to the intermediate transfer belt 10.
The transfer roller 20 is provided to selectively contact the guide roller 14 with the intermediate transfer belt 10 therebetween. Accordingly, the toner image transferred to the intermediate transfer belt 10 is transferred to printing paper P passing between the intermediate transfer belt 10 and the transfer roller 20. Further, the transfer roller 20 is supported by a transfer panel 2 that is capable of pivoting to open and close the printer body frame. As a result, in the event that the paper P is jammed in a paper path between the intermediate transfer belt 10 and the transfer roller 20, it is possible to easily remove the jammed paper by opening the transfer panel 2 from the printer body frame.
The transfer roller separating/pressurizing unit 30 separates or contacts the transfer roller 20 and the intermediate transfer belt 10. Here, in the case of separating the transfer roller 20 and the intermediate transfer belt 10, the toner image of the photosensitive drum 1 is transferred to the intermediate transfer belt 10. In the case of pressurizing and contacting the transfer roller 20 and the intermediate transfer belt 10, the toner image of the intermediate transfer belt 10 is transferred to the printing paper P. In the color laser printer, the intermediate transfer belt 10 rotates four times to form one color image. Therefore, while the intermediate transfer belt 10 makes four revolutions, the transfer roller 20 and the intermediate transfer belt 10 must be separated from each other, and for this reason, the transfer roller separating/pressurizing unit 30 is required.
The transfer roller separating/pressurizing unit 30 includes a cam unit having a pressurizing block 31 to support the transfer roller 20 to be rotatable, a separating lever 32, of which one end is connected to the pressurizing block 31 and the other end is rotatably positioned by a hinge 32a on the printer body frame, and a tension coil spring 33 to elastically support the separating lever 32 in a counter-clockwise direction to maintain a separation state of the transfer roller 20 and the intermediate transfer belt 10, as shown in FIGS. 2A and 2B. The tension coil spring 33 is positioned between the separating lever 32 and the transfer panel 2. The separating/pressurizing unit 30 further includes a cam 34 to contact the transfer roller 20 against the intermediate transfer belt 10 by rotating the separating lever 32 clockwise; and a pair of compression coil springs 35 being provided between an axis of the transfer roller 20 and the pressurizing block 31 to elastically support the transfer roller 20 to the side of the intermediate transfer belt 10, to form a constant transfer nip therebetween when pressurizing the transfer roller 20 with the cam unit.
In the transferring apparatus of the general color laser printer described above, when the toner image of the photosensitive drum 1 is transferred to the intermediate transfer belt 10, the cam 34 is positioned as shown in FIG. 2B. The separating lever 32 is rotated counter-clockwise about the hinge 32a at a predetermined angle by the recovery force of the tension coil spring 33 and this enables the transfer roller 20 and the intermediate transfer belt 10 to be separated at constant intervals.
Furthermore, at the point that the transfer of the toner image of the intermediate transfer belt 10 is completed, the cam 34 is positioned as shown in FIG. 2A. Here, the separating lever 32 is rotated clockwise around the hinge 32a at a predetermined angle by overcoming the recovery force of the tension coil spring 33, thereby maintaining the state that the transfer roller 20 and the intermediate transfer belt 10 closely contact each other. While the transfer roller 20 is regularly pressed against the intermediate transfer belt 10 by the recovery force of the compression coil spring 35, a transfer nip is formed.
Then, the paper P is fed between the closely contacted transfer roller 20 and the intermediate transfer belt 10 and the toner image transferred to the intermediate transfer belt 10 is transferred to the paper P.
In the transfer process as described above, the transfer nip between the transfer roller 20 and the intermediate transfer belt 10 must be evenly maintained because this nip greatly affects the quality of an image.
Furthermore, the transferring apparatus of the color laser printer as described above is installed on the transfer panel 2 which is capable of pivoting to open and close the printer body frame to remove the paper jammed between the transfer roller 20 and the intermediate transfer belt 10. Therefore, when mounting the transferring apparatus after opening and closing the transfer panel 2, the position of the transfer roller 20 with respect to the intermediate transfer belt 10 is changed and thus the required transfer nip is frequently not formed.
More specifically, as illustrated in FIG. 1, the transfer roller 20 has degrees of freedom in X, Y and Z axis directions with respect to the intermediate transfer belt 10. The degree of freedom in the Z axis direction is compensated for since the length of the transfer roller 20 is longer than that of the paper. The degree of freedom in the X axis direction is compensated for by correcting the recovery force of the compression coil spring 35. However, as to the degree of freedom in the Y axis direction, there occurs a case in which the intermediate transfer belt 10 is not exactly identical with the transfer roller 20. This is because the position of the intermediate transfer belt 10 is determined in the printer body frame while the Y axis direction position of the transfer roller 20 is determined in a different unit (the transfer panel 2), being assembled with the hinge 32.
Therefore, in the conventional transferring apparatus, since the degree of freedom in the Y axis direction of the transfer roller 20 with respect to the intermediate transfer belt 10 is not properly restricted, it is impossible to maintain an even transfer nip and problems such as non-uniform image, image loss, inconsistent image density and paper crumpling occur.